Fix this/these issue on page 218

Details

Description

-While the instructions below will work with Nexus Open Source, this instructions
-assume the filesystem of Nexus Professional. If you are missing Jetty JAR files, you
+While the instructions below will work with Nexus Open Source, these instructions
+assume the filesystem used by Nexus Professional. If you are missing Jetty JAR files, you
should obtain them from the Jetty project page: http://www.mortbay.org/jetty/.